1. Field of the Invention
The present invention relates to the field of electro-optical liquid crystal display materials and in particular, to a polymer stabilized alignment type liquid crystal composition with good stability and a liquid crystal display device using the same.
2. Description of Related Art
Liquid crystal display devices are used in household appliances represented by clocks and electronic calculators, measuring instruments, automotive panels, word processers, computers, printers, televisions or the like. For night scene, typical display modes include PC (phase change), TN (twist nematic), STN (super twisted nematic), ECB (electrically controlled birefringence), OCB (optically compensated bend), IPS (in-plane switching), VA (vertical alignment), CSH (color super homeotropic). Depending on the way in which the devices are driven, they can be divided into PM (passive matrix) type and AM (active matrix) type. PM is further divided into static type and multiplex type. AM is further divided into TFT (thin film transistor) type and MIM (metal insulator metal) type. TFT includes amorphous silicon type and polycrystalline silicon type. The later can be divided into high-temperature type and low-temperature type depending on the manufacturing process. Depending on the type of a light source, the liquid crystal display devices are divided into reflective type with natural light, transmissive type with backlight, and semi-transmissive type with both natural light and backlight.
Among these display modes, IPS, ECB, VA or CSH is different from commonly used TN or STN in that the former uses a liquid crystal material with negative dielectric anisotropy. Among these display modes, particularly AM-driven VA type, for applications requiring a display device with high speed and wide view, it is expected that liquid crystal display devices are applied in TV.
Like IPS mode, VA mode is normally black. The difference between the two modes is that in a VA mode panel, a negative liquid crystal is used as liquid crystal molecule in a liquid crystal layer, and transparent electrodes are disposed on upper and lower substrates to create an electric field perpendicular to the substrates. In non-powered conditions, the major axis of the liquid crystal molecule is perpendicular to the substrates to form a dark state; and in powered conditions, the major axis of the liquid crystal molecule is horizontally parallel to the substrates. The rubbing on the substrates is required for its initial alignment, which results in contamination, static electricity and pre-tilt angle being difficult to control. In order to address the initial alignment problems of VA mode, various derivative modes have been developed, such as MVA (multi-domain vertical alignment), PVA (patterned vertical alignment) and PSVA (polymer stabilized vertical alignment). Among theses, PSVA is becoming dominant for its high transmission, high contrast and fast response.
In PSVA mode, a polymerizable compound is used to control the arrangement direction of the liquid crystal molecule by applying an electric field to place the liquid crystal in a desirable arrangement state and performing UV exposure while maintaining at this state to polymerize the polymerizable compound in the mixed liquid crystal, thereby “curing” the desirable arrangement state of the liquid crystal.
No rubbing alignment process is required in the PSVA mode, such that unwanted problems of contamination and static electricity caused by rubbing in the TN and IPS modes can be avoided. However, since the polymerizable compound used in PSVA may polymerize due to light or heat, “unexpected” polymerization should be prevented during manufacture, storage, transportation and use of PSVA liquid crystals, which otherwise will significantly affect the final alignment of liquid crystals.
Also, the negative liquid crystal composition used in PSVA is required to have a larger absolute value of dielectric anisotropy (|Δ∈|) in order to reduce the drive voltage, thereby meeting the requirements of low-carbon and energy saving.
A liquid crystal medium is disclosed in Chinese patent CN03146257.X, which comprises a negative liquid crystal monomer having a specific structure and a polymerizable compound. However, method and materials for preventing “unexpected” polymerization have not been mentioned therein.
A liquid crystal mixture for use in polymeric alignment is disclosed in Chinese patent CN200710148981.4, which comprises a negative liquid crystal monomer having a specific structure, a photo-polymerizable or thermally polymerizable monomer and an inhibiter having a specific structure. Although the stability of the liquid crystal mixture is improved, the absolute value of dielectric anisotropy (|Δ∈|) is too low, which cannot meet the requirements of low drive voltage.
Thus, there is an urgent need for a new liquid crystal composition for use in the PSVA mode, which can not only provide a larger absolute value of dielectric anisotropy, but also effectively inhibit “unexpected” polymerization.